The present invention relates in general to automotive air bag systems, and, more specifically, to a hidden air bag deployment door formed by an instrument panel substrate and a molded air bag chute.
Air bag deployment chute assemblies have been put into commercial use for the passenger side of an instrument panel of automotive vehicles. The chute assembly couples an air bag module (typically containing a folded canvas bag and chemical propellants for inflating the bag on command) to a door support panel or substrate of the instrument panel. A typical structure for a chute assembly includes a tubular outer chute wall, one or more door flaps, a flange surrounding the door area, and one or more hinge members or areas connecting the door flap(s) to the outer wall and flange.
For styling purposes, it is desirable for the air bag deployment door in the instrument panel to be invisible when viewed from the passenger compartment. In other words, the visible or “Class A” surface of the instrument panel is preferably seamless. Therefore, a pre-weakened seam is required in the substrate (on the “Class B” side) to facilitate tearing open of the door during air bag deployment. In order to prevent torn or severed pieces of the substrate from being expelled into the passenger cabin, the door flap(s) of the chute attach to the substrate door area so that the door flap and hinge act as a tether. A common method to attach the chute door flap and flange to the substrate has been by plastic welding, such as vibration welding, hot-plate welding, and the like.
A typical passenger air bag door is designed to transfer airbag pressure loading created by the expanding air bag onto the tear seam to release the door as quickly as possible during a deployment. A clean and quick separation of the tear seam helps avoid material fragmentation during airbag deployment. A stiffer door can transfer the airbag loading to the tear seam quicker than a more flexible door, and results in less risk of fragmentation. Thus, one of the challenges in passenger air bag door design is to develop a chute and door system wherein the door has enough stiffness to effectively transfer the airbag deployment force to the tear seam while keeping both production costs and weight low.
A typical air bag chute structure is formed as a one piece thermoplastic injection molding combining a flange or collar for attaching to an instrument panel substrate, a door connected to the flange by a hinge, and a chute for connecting to an air bag module and providing a deployment path to the door. A common material is TPO (Thermoplastic Olefin) such as Dexflex™ or other materials that exhibit superior ductility at very cold temperatures at least to −30° C. and good toughness at high temperatures at least to 90° C. (e.g., TPE or TEO). Since the materials are somewhat flexible, door stiffness has been increased by adding ribs and/or increasing the material thickness. Since the chute assembly is injection molded, it is necessary to avoid a die-lock condition during the molding process—which limits the range of compatible rib shapes and sizes that can be used. Thus, the known methods may not achieve the desired stiffness and may result in other disadvantages such as increased cost and weight (e.g., an undesirable stiffness-to-weight ratio).
Because of the relative positions of an air bag module and the deployment door, and the orientation of the styled surface of the instrument panel, the resulting impact force of the expanding air bag against the door is slanted with respect to a direction perpendicular to the door. The invention further recognizes that the known orientations cause a shortening of the moment arm of the resultant force (with a concentration of force close to the door hinge) and that it would be desirable to increase the moment arm.